Christopher Eppig in Scientific American:
A great deal of research has shown that average IQ varies around the world, both across nations and within them. The cause of this variation has been of great interest to scientists for many years. At the heart of this debate is whether these differences are due to genetics, environment or both.
Higher IQ predicts a wide range of important factors, including better grades in school, a higher level of education, better health, better job performance, higher wages, and reduced risk of obesity. So having a better understanding of variations in intelligence might yield a greater understanding of these other issues as well.
Before our work, several scientists had offered explanations for the global pattern of IQ. Nigel Barber argued that variation in IQ is due primarily to differences in education. Donald Templer and Hiroko Arikawa argued that colder climates are difficult to live in, such that evolution favors higher IQ in those areas. Satoshi Kanazawa suggested that evolution favors higher IQ in areas that are farther from the evolutionary origin of humans: sub-Saharan Africa. Evolution, the hypothesis goes, equipped us to survive in our ancestral home without thinking about it too hard. As we migrated away, though, the environment became more challenging, requiring the evolution of higher intelligence to survive.
We tested all these ideas.